A joint of casing typically includes threaded couplings at either end. These threaded couplings allow two joints of casing to be screwed or threaded together. Generally, a joint of casing has a male thread on one end of the casing with a corresponding female thread on the other end. There are various types of threads depending on the requirements of strength and the type of casing.
Initially, the process of handling or running casing is not very different from running drill pipe. Once the joints of casing are brought to the site, they are inspected and measured. The casing joint is then taken up the ramp to the drill floor, latched to an elevator, suspended from the travelling block by two equal length slings or steel cables, and then hoisted by the travelling block until the casing is hanging vertically. After lowering the joint through the rotary table, the drill crew then places the slips around the first joint of casing to secure it to the master bushing of the rotary table. The slips now suspend the casing string in the hole. Because the hole in the rotary floor is slightly tapered, the slips act as a wedge, holding the casing vertically in place by friction. Slips support the casing within a conical bushing. Subsequent joints of casing are then stabbed and screwed into the secured casing below to form the casing string.
The process of stabbing is somewhat of an art because aligning the casing properly is both very difficult and important. Although the diameters of the casing are relatively large, the threading on each can be quite fine. As a result, the casings are very sensitive to alignment and threading. The act of stabbing is generally performed by a derrickman located on a stabbing board. The stabbing board is a platform that is normally located about 40 feet above the drill floor, but generally it can be moved up or down depending on the length of the casing and other circumstances. The derrickman on the stabbing board holds the hanging casing joint and positions it over the secured casing below. Generally, crew-members on the drilling deck, such as the tong operators, direct the derrickman on the stabbing board to align the casing. The tong operator(s) then aligns the threads of the casing and couples them together using a pair of casing tongs. These casing tongs are hydraulically powered and clamp onto the casing with jaws. The tong operator can use the casing tongs to rotate the hanging casing and thread it into the coupling of the secured casing below. Proper make-up of the torque is critical for a good connection. During the process of threading one piece of casing to another piece of casing, lifting elevators are attached to the casing load, which consists of the casing string or casing assembly. The slips are released and the casing load is lowered further down into the hole by the elevators. The slips are once again attached to secure the casing load, and the process of adding casing is repeated. Generally, a single-joint (transfer) elevator is used to hoist and position the next piece of casing to be stabbed into the secured casing assembly (or casing load) below while the slip-type (lifting) elevator is used to hoist the entire casing load.
The conventional method of stabbing casing has many inherent risks. There are several hazards associated with having to have a derrickman perform the stabbing operation on the stabbing board. The stabbing board is suspended approximately forty (40) feet in the air and as a result, the derrickman is exposed to the risk of falling or being knocked off the platform by various equipment. In addition, there is a risk of falling while climbing to or from the stabbing board. Although the stabbing board serves only one purpose, it remains an obstacle to other equipment in other operations. Even though the stabbing board can be folded up, it can still snag or catch nearby equipment. Further, because the stabbing board is fairly complicated and because it must be positioned to avoid completely blocking other equipment and operations, the land rig crew spends a considerable amount of time setting up and breaking down the stabbing board.
Other problems with the conventional method of stabbing casing stem from the use of the transfer elevator. Use of the transfer elevator to hoist and position the joint of casing to be stabbed is a slow and cumbersome process and involves several manual steps. The drilling rig environment is a hazardous one, and the more manual steps involved in a given process, the greater the likelihood of damaged equipment and injury to the crew. In addition, the transfer elevator presents several possible hazards. The transfer elevator supports the casing joint with relatively light slings. These slings do not allow the operator to control how the casing joint will hang. As a result, there is a real possibility that the casing joint will snag on a piece of equipment as it is hoisted up by the transfer elevator. Because the transfer elevator is powered by the rig's drawworks, there is more power associated with the transfer elevator than there is capacity to hoist. Therefore, if the casing joint does get snagged on a piece of equipment, the slings are prone to being pulled apart by the excessive power and the casing joint will drop.
Increasingly, drilling contractors are using top drive systems. A top drive is a drilling tool that hangs from the traveling block, and has one or more motors to power a drive shaft to which crewmembers attach the drill string. Because the unit's motor can rotate the drill string, no Kelly or Kelly bushing is required. The top drive unit also incorporates a spinning capability and a torque wrench. In addition the top drive system has elevators on links. The conventional method of handling casing requires the use of casing tongs, a costly contract service. The tong equipment generally also requires an outside crew to operate them. Given the power and control of the top drive, it is desirable to use the top drive system to replace the expensive services of the tong operators. In addition, it would be desirable to eliminate the need for a crewmember on a stabbing board and use of slings on the transfer elevator in the casing stabbing process.